The present invention relates generally to polymer materials, in particular to polymers which are resistant to the growth of certain microbiological species such as bacteria and fungi. In particular, the present invention relates to sheets of acrylic polymers that are thermoformable.
The acrylics group of polymers is dominated by two resins—one used principally for blending with other resins and as a fiber (polyacrylonitrile or PAN) and the other used principally for molding (polymethylmethacrylate or PMMA). The present invention is directed primarily toward PMMA.
The molding resin, PMMA, is a very popular engineering thermoplastic material. Common brand names for PMMA include Perspex®, Plexiglas®, Lucite®, Acrylite®, ModenGlass®, and Diakon®. The resin is polymerized by the addition polymerization method and forms a plastic that is atactic and therefore amorphous.
The most important property of PMMA is its optical clarity. This plastic has a very high light transmittance. It is also quite insensitive to UV light. It has low oxidation sensitivity, a high gloss, and overall weather resistance. Together, these characteristics result in a high retention of clarity and light transmittance over long periods of time. These desirable optical properties led to numerous and diverse applications such as windshields (especially for aircraft), skylights, outdoor signs, boat surfaces, automobile tail lights, display cases, light fixtures, shower stalls, spas, bathroom basins, and counter tops, hot tubs, shelving and decorative laminates, among others.
The relatively low processing temperature, low shrinkage, and good dimensional stability make PMMA easy to process in injection molding and extrusion. A major product for PMMA is acrylic sheet which can be thermoformed into many of the products mentioned earlier.
The popularity of acrylic sheets in these applications also means that acrylic sheets are often exposed to high levels of moisture. In the areas of baths, showers, and spas the acrylic material is almost constantly in contact with water. This is especially the case with spas and hot tubs which have considerable fluid volume and are therefore not drained on a regular basis.
Water left in bath basins or spas for only a couple of days can become fouled with numerous biological organisms. In many instances a yellow or brownish scum line develops on the surface of the basin or spa near or at the interface of the standing water and air. With additional aging the water becomes cloudy as algae, bacteria and fungi grow.
Even in areas where water does not stand for extended periods of time, e.g., bathroom sinks and basins, the frequency of wetting can lead to substantial bacterial and fungal growth.
In short, thermoformable acrylic sheeting is often used in applications having high moisture exposure. Thus acrylic sheeting can serve as a growth surface for bacteria, fungi and other microbes that are aesthetically unpleasing, damaging to the product (e.g., cause staining or discoloration), and/or harmful to human health. Accordingly, there is a great need for a control strategy for successfully reducing or substantially eliminating the proliferation of microbes on acrylic surfaces.
The majority of existing control strategies for reducing microbes on acrylic surfaces utilizes treatment of the water by application of chemicals or topical application of antimicrobial agents. For example, in swimming pools and large hot tubs, the algae and the bacteria are usually controlled by the addition of an oxidant such as sodium hypochlorite or an in situ generation of ozone, and by filtering the water through diatomaceous earth. Such treatments are expensive and in small applications, such as a bathroom basin, they are not an option. Bathroom basins and smaller hot tubs and spas typically require the application of topical antimicrobial solutions (e.g., bleach) followed by physical abrasion to remove built up bio-scum. Such topical treatments are time consuming and are not durable.
What is desired is a thermoformable acrylic sheet that has built-in antimicrobial protection that reduces or substantially eliminates the proliferation of bacteria, algae, fungi, and other microbes on its surface. Such an acrylic sheet would also reduce and/or substantially eliminate the need for exterior treatment of the sheet or water.
Attempts at producing such sheeting are known from the prior art. For example, international publication WO 99/47595 discusses a biocidal plastic material comprising an acrylic polymer containing 5% to 50% of a rubbery co-polymer and a biocidal compound. The polymer is purportedly suitable for use in preparing extruded sheets for thermoforming applications. Several biocides are discussed including triclosan, silver, isothiazolones, zinc pyrithione, 10-10′ oxybisphenoxyarsine (OBPA), and benzisothiazolin-3-one derivatives.
Similarly, European Patent Application EP 893,473 discusses a thermoplastic acrylic sheet composition that can contain an antimicrobial composition. Trade names for OBPA and isothiazolones are mentioned as possible antimicrobial agents. The '473 document, however, provides no guidance regarding effective amounts of antimicrobial agents or how to incorporate them into the acrylic polymer.
Although some of the known acrylic sheets having built-in antimicrobial agents demonstrate some efficacy against the buildup of microorganisms, there is a continuing need for more efficacious antimicrobial sheeting. The reason for this continuing need is three-fold.
One reason is based in economics. The addition of some antimicrobial products into acrylic polymers increases the per-unit cost of sheeting to levels that are unacceptable to the consumer. The use of silver as an antimicrobial agent is a notable example.
Another reason is based in manufacturing problems. Most industrial acrylic sheet manufacturing processes are precisely controlled processes that produce product with specific characteristics (e.g., optical clarity). The addition of antimicrobial agents often alters the process (e.g., curing time) and/or results in unacceptable product (e.g., opaque sheeting). Inorganic antimicrobial agents such as silver and copper are notable examples in that they tend to discolor thermoformed articles.
Finally, fungal growth remains a problem in spa and bath applications.
Accordingly, there is a need for a commercially acceptable solution to the above discussed problems. This solution should provide an economical alternative to existing antimicrobial acrylic products. This solution should also integrate into existing acrylic sheet manufacturing processes without causing unacceptable process changes. Finally, the solution should demonstrate acceptable efficacy against fungal growth.